"Shutter speed" is the time the shutter is open during an exposure. It is usually expressed in seconds or fractions of a second: 1 s, 1/2 s, 1/4 s ... 1/250 s, 1/500 s. Fast shutter speeds reduce the amount of light that reaches the image sensor, while slow shutter speeds increase it.
Changing the shutter speed changes the appearance of moving objects. Fast shutter speeds freeze motion, while at slow shutter speeds blur caused by camera or subject motion may be visible. In other words, fast shutter speeds reduce subject and motion blur.
On the other hand, slow shutter speeds let you capture the motion of flowing water and other objects. Adjusting shutter speed lets you control how motion is expressed.